Invincible
by Super Vanilla Bear
Summary: "You know Casey. He's invincible." But what happens when that just isn't true anymore?
1. A Sick Day's Night

**Author's Note: **I do not own the television show _Chicago Fire_ or any of its characters.

This is my first crack at a multi-chapter fanfic for this series, so I hope it goes well. Just so you know, the first chapters may seem a little slow because I can't jump right into it, otherwise I doubt many people will understand. I hope everyone here loves sick fics as much as I do. Be prepared for bromance and a lot of Casey whump! I hope everyone will enjoy this!

* * *

_A Sick Day's Night_

"Hermann, what the hell are you doing?" Otis hollers from the roof of truck eighty-one, laying flat on his stomach to see what is going on outside. Despite the below freezing level temperatures, Hermann decided to invest in a huge grill and barbecue steaks and hamburgers tonight for the house. Otis rolls his eyes, but stands up and proceeds to scamper down the ladder, zipping up his coat as soon as he steps into the arctic of Chicago.

Christopher Hermann is bundled underneath at least a dozen layers of protective clothing, his face bright red from the wind. His hands are trembling as he flips a few of the burgers, and he wishes more than anything in the world that he hadn't agreed to do this for the guys tonight. It's January, and the high for today was only twenty-one degrees. He sighs heavily, but smiles as soon as he sees Otis make his way over to him.

"You're gonna get frostbite out in this, genius."

"Hey, it's not my fault that the guys want steak."

"No, but it is your own idiotic fault for buying this thing and offering," Otis says, patting the giant grill on the top. He's starving, and he wants nothing more than to sink his teeth into a big, juicy hamburger, but considering it's Hermann and not Mills cooking, this will be interesting. "Watch it! They're starting to burn!"

Hermann shrugs. "Well, it's a good thing we're firefighters then, huh?"

Out of the smoke, both men see a figure immerge. While Otis's heart drops a bit in anticipation, Hermann goes straight back to barbequing. Otis breathes out a sigh of relief when he sees Matt Casey walk toward them. He is obviously uncomfortable in the chill of Illinois, and Otis can tell from ten feet away that he's shivering uncontrollably.

"Morning, Lieutenant," he offers with a smile.

Casey doesn't return a grin in the slightest. "Morning," he huffs, pushing his way past him and heading inside. His fingers quiver as he fumbles with the zipper of his coat, coughing quietly under his breath. His lungs ache for a breath of air that isn't strained, but his asthma flares up instead. Casey quickly opens the door into the house, wrapping his hand around his inhaler in his pocket, planning to immediately head into his office since it doesn't look too busy in here right now. He frowns when he sees Dawson walking toward him, a bit of left of anger stewing inside him. He ducks away, closing the door to his quarters, hoping she doesn't follow him.

His breathing is too rapid and shallow, every gasp of air stings. He's thankful when he puts the canister into his mouth and administers support for his troubled lungs. The fresh use of his medication provides fast relief. Casey rubs his hands down the side of his newly clean shaved face, exhaling before opening his eyes.

Crap.

Dawson is standing at his door, a worried expression completely taking over her face. "Are you okay?" she asks as soon as she barges in. She's never seen Casey use an inhaler before. She didn't even know he had asthma. He's as pale as a blank piece of paper, but there are deep purple bags beneath his bloodshot eyes. She watches him sink away, sitting down at his desk and turning around, ignoring her presence. "Are you really still mad at me?"

In light of her brother's shooting, she had to make a deal with Voight, the cop that gave Casey so much trouble that he finally managed to put behind bars a few months ago. She _had _to do it. She still doesn't understand why Casey is still so pissed at her for something that she did a week ago. He won't say more than five words to her, won't look in her general direction, and won't acknowledge her period anymore.

"Casey?"

"What do you want?" he questions bitterly. He isn't in the mood for any of this.

"I want you to stop being so pissy and talk to me."

Casey decides it will be best if he just continues to write out his log from the past week, so that's what he does. He doesn't want to be so angry with her, but how can he not be? She just put his life in danger… again. Now, he has to worry about his mom and sister being dragged into this mess too. He sighs when he hears Dawson slam the door to his quarters closed with great force, but he's too exhausted to protest. Instead, he lays his down on his desk, padding it with his arms.

He's asleep in an instant.

* * *

Chief Boden storms out of his office two hours later. "Severide, my office now." He doesn't scream it this time; he just says it. Severide sheepishly cowers away, just like he always does when the Chief is clearly mad. Surprisingly, he doesn't even look at him while he's leaving. "Anyone seen Casey?" Mills points down the hall, but Boden is too steamed to even thank the candidate with a simple nod.

What Boden sees next is something he never would have thought he would witness. Matt Casey is hunched and passed out on his desk, his head lolled to the side. His mouth is hanging open, and he can hear how quick of breaths he's taking. He's never seen Casey so out of it, even when he works double shifts and can hardly stand up straight. There's something that's not right about this at all. For someone who just had a day off, he seems oddly unrested. Plus, the tinge of green Boden sees on his cheeks is sickening.

"Casey?" he calls, not really expecting an answer. Part of him almost feels bad for shaking him awake, but he has to get to the bottom of this. Casey blinks a few times before sitting up, one hand placed firmly over his stomach. "Matt, you alright?" The younger firefighter wordlessly nods. Boden's hand is still on Casey's shoulder, and his heart drops when he feels the blond trembling beneath his touch. "I need to speak with you in my office. Now."

With how wilted Casey looks, Chief Boden almost considers moving the meeting in here, but he ignores his gut. Either Casey or Severide is in huge trouble right now, and he needs to figure this situation out. Casey stands up, his legs quivering like jelly as he makes the walk that feels like miles down to Chief's office. By the time he collapses in a chair beside Severide, his brain is pounding viciously into his skull.

"I just got off the phone with the police department. Apparently, there was a bit of a scuffle between one of the cops and one of our men at the scene two days ago. That cop is now in the hospital for a recently diagnosed concussion. Do either of you know anything about that?" In theory, Boden should honestly know, but he wasn't present at that fire due to his ex-wife calling about his son.

Severide and Casey glance at each other without speaking.

"Um, Chief, if this happened two days ago, why is it just now being brought up?" Severide inquires.

Boden glares at the brunette. "Because this man is in the hospital, and the police department is now ten feet up my ass."

Normally, Severide would laugh at that, but there isn't any time for it. "Chief, I may or may not have been involved with…"

Suddenly, there's a cough emanating from Casey next to him. It's harsh, wet, and sounds like it's bubbling up from the pit of his stomach. Without saying another word, Severide places a trashcan in front of the slightly younger man. He's heard that gurgling, dying whale noise one too many times. He's taken care of Shay when she's been sick before, and her illnesses start like this. He knew Matt didn't look too good when he came in this morning. He ignores the fact that Casey blew up on him the other night for the fight Chief is talking about and places his hand on Casey's shoulder.

"Matt…"

Casey's brain feels like mush, and he just needs to sleep. He slams his head forward, barely able to keep his eyes open, resting it on to the top of the trashcan cradled to his chest. He doesn't feel like he's going to throw up anymore; he's just so tired. Severide's warm hand on his back isn't helping either.

"Okay, can we talk about this another time?" Severide asks, irritated and freaked out.

"Actually, this is something we need to discuss…"

"Chief, he looks like he's gonna pass out!"

Carefully, Severide helps Casey stand, who is already half asleep. He walks him down to his quarters, shrugging off the awkward stares from everyone in the café area. Thankfully, there's a bed in his office, waiting for him patiently. Severide places a nearly comatose Casey in bed, covering him up with the one thin blanket on there.

The second he hears Matt snore, he feels better.

* * *

Casey wakes up with tears streaming down his flushed cheeks. He can't breathe out of his nose, his throat feels like it's almost entirely swollen shut, and he's shaking so hard that he can't concentrate on anything else. He doesn't know how he woke up crying, though. He chalks it up to a bad dream, but he isn't sure. Casey quickly rubs the tears away, determined and knowing he shouldn't even be like this because he's a little under the weather. Normally he's fine when he's sick. Sure, he's a cold and achy, but it's nothing he can't handle. It's just that, this time, he feels… different.

He stands up, coughing and sniffling as he grabs for his thick winter coat to throw over his other jacket, sweater, and thermal long sleeved shirt. He pads down the hallway in only his black socks, careful not to make any unnecessary sound. Through blurry vision, he can make out that the clock says it's just past two in the morning. No one in the house is up. In order to try to clear his throat, he gulps down an entire glass with ice cold water, not even caring that it hurts just as much as it helps.

Casey rubs his tender neck, noting that his lymph nodes are too swollen to even touch. He coughs as quietly as possible, feeling nausea bubble up in his stomach. _C'mon, Matt. Be a man_. Without thinking anything else about himself feeling off, he collapses on to the old, creaky couch, stretching out his legs when he lays down. It doesn't take long for his body to, once again, dissipate in exhaustion.

* * *

It's only four in the morning when Severide pulls himself out of bed, his hunger pains too extreme to ignore. "Jesus Christ, be quiet already," he tells his stomach, which has been making the most God awful noises in the entire world for the past twenty minutes. He stomps down the hallway, loving the sensation of his bare feet hitting the cool tile below, searching for some beloved steak that was provided by Hermann this evening.

When he enters the rec-area, his heart drops into his stomach. Matt is curled in on himself on the couch, trembling and snoring at the same time. Kelly brushes away his desire to eat and grabs the electric blanket off of the back of the sofa. He places it over Casey's sleeping form, plugging it in just seconds later.

"Jesus, Matt," is all he can manage to say. He's never seen Casey in such a vulnerable state before. The stubborn bastard would usually just hide the fact that he feels like shit, but this is entirely new for Severide. Matt is normally such a hard ass that no one can ever even pull their wits enough together in order to comprehend that he's sick.

Severide kneels down on the floor to feel his forehead, but he is immediately caught off guard by the blood dripping out of Casey's nose.

Oh shit.

* * *

**Author's Note: **So, what did you think? What do you think is wrong with Matt? I guess you'll just have to tune in to figure that out. I'm terribly sorry if everyone was out of character. Don't worry, the tension between Severide and Casey isn't completely gone yet. I wanted Kelly to see that Matt is struggling though. And, this won't be entirely just a sick fic. I have plenty for Severide himself to go through in this as well. Opinions would be greatly appreciated! Thank you so much for reading, I hope you enjoyed it, and please remember to leave a review!


	2. Nothing's Wrong

**Author's Note: **I do not own the television show _Chicago Fire_ or any of its characters.

Thank you so much for the positive reviews on this! I was extremely thrilled to find fifteen new reviews today, as well as multiple alerts and even a few favorites. Thank you so much, everyone! You have no idea how much I really do appreciate it. Your feedback has an impact on how I write this story; so if you ever have any suggestions on what I could do to make this story even better, please feel free to let me know. I promise I won't get mad or bite. Once again, thank you!

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_Nothing's Wrong_

Oh shit.

A glob of crimson is pooling around Casey's upper lip. His nose isn't bleeding a lot by any means, but it sure as hell made Severide's heart drop. The brunette scrubs his hand down the side of his stubbly face, sighing as he grabs a tissue to wipe it up. Usually, Casey is an extremely light sleeper, which he learned quickly after trying to prank him several times with Andy. But now, Casey doesn't even budge when Severide cleans the blood off of his face. "Dammit, Matt," is all he whispers, too distraught to say much else.

He has no idea what is going on with the younger firefighter, but he'll be damned if he leaves him in here to wake up alone. If he were sick, he'd want someone there too. Shay always babies him when he doesn't feel well. No, he's not going to baby Casey, but making sure he's okay isn't bad. Is it? The worry overrides the bit of resentment and anger still pent away at him. Casey… He put Darden through that window. If he hadn't…

Severide brushes off those thoughts immediately, using his thumb to smear the few stray tears that had formed just by thinking about Andy. _Be a man, Kelly_ is what his mind tells him to do, so that's what he does. He silently sits down in a chair next to Casey's head, propping his feet on top of the coffee table. On a normal night in the house, he would have woken Casey's sorry ass up, but he can't even fathom that now.

* * *

"What the hell?"

Leslie Shay walks into the rec-area at just past seven in the morning, hair tied in a messy ponytail and yawning, not expecting to see this… odd sight. Kelly is sprawled out on the length of a chair, practically stretching out on to the table, his mouth hanging wide open and snoring like a banshee. Typical Kelly. She's used to that anyway. But, what she isn't used to is Casey being curled in a ball underneath an electric blanket with flushed cheeks and deep, dark purple bags beneath his eyes, even though he's sleeping.

Aw…

She immediately recognizes what has happened, and she smiles happily. Kelly was looking out for Casey. While Kelly will come home swearing up and down that Casey has turned into an inconsiderate asshole, she knows he still cares about him. Why else would he have been so worried and wanting him to always been in his presence during the Voight episodes? No matter what he says, Shay knows the truth. The two of them act like they hate each other, but they both still sympathize.

Shay jokingly kicks Kelly's foot, sending him flailing like a fish out of water. "What?!" he screams groggily, a look of disgust written on his face.

"Morning, sunshine. What's wrong with Casey?" she questions.

Severide groans, stretching out his sore limbs as he starts to talk. "I think he's sick. He almost passed out during a meeting with Chief yesterday and won't stop shaking. He's been doing it all night."

"You've been out here all night?" Shay asks with a grin.

Severide rolls his eyes. "Shut up." He's not laughing though; he's a little annoyed. Casey pisses him off so badly sometimes, but yet Severide still can't just let him be a sick brat on his own. And now Shay is going to make a big deal out of nothing, but Severide himself isn't sure that it's nothing. Shaking, a bloody nose, and being so exhausted that you can't stay awake has to be a sign of something.

Shay can tell Kelly isn't in the mood for joking, so she kneels down in front of Casey instead, not wanting to intrude in his personal space by touching his forehead, so she gently taps him on the shoulder. Not to her surprise, his bloodshot eyes open, revealing a mess of a Casey. He moans, but nothing really comes out. Shay's heart rate starts to elevate, and she bites her bottom lip. "Matt, are you okay?"

Casey nods, the movement itself hurting his throat so much that he can feel tears prickling at his eyes. He pulls the blanket up closer, burying his face within the fabric. He doesn't want everyone to see him like this, so he has to come up with some. Fast. He rubs his eyes underneath the electric comforter, trying to get rid of some of his sleepiness, determined to act as if nothing is wrong. There's no way he's dealing with the entire house being freaked out about him being a little run down.

The freezing morning air hits him like a ton of bricks as soon as he removes the blanket and sits up, a rush of dizziness slamming into him full force. However, he has perfect posture, a typical Casey thing to have. He begins to stand up when Severide pushes him back down. "What was that for?" he inquires, trying his hardest not to swallow harshly, which would be a clear indicator of his sore throat. His voice is shot, and he needs to find an excuse as to how he lost it so they will leave him alone.

"You're sick. You don't need to be moving around," Severide explains.

Casey raises his eyebrow, not convinced. "No I'm not."

"Look, I'm not having this conversation with you, Matt. You know you are. Just sit there and we'll call a cab for you or something."

Casey gets to his feet despite his weariness. "I'm fine, Kelly. Nothing's wrong."

The second he stands up, his vision goes entirely black. He uses his fingers to put pressure on both of his eyes, trying to steady his breathing as he does so. He's so dizzy that he can't move without feeling like he's going to fall over at any given moment. He flinches when he feels someone's hand on his shoulder, thanking God when he can finally make out Severide and Shay's worried expressions.

"Yeah, you look real fine there, Casey," Severide says.

Casey clenches his jaw, biting down on his bottom lip. "Look, Severide, I said I'm fine. There's nothing wrong. Yes, I'm tired, but I'm not sick. Don't treat me like I'm an invalid." With that, he walks down to his office, closing the door quietly. He's not mad; he's just a little aggravated that no one will leave him alone about this. He's run down, but he will feel better once this shift is over. The only things that are even bothering him right now are his throat and how cold he is.

"_Squad 3, truck 81, ambulance 61… house fire at 74 East Mildred Road."_

Dammit.

* * *

The house is completely engulfed in flames. Casey sniffles at the smell of gasoline burning. It doesn't seem like a normal house fire period. Thick, black smoke erupts from the roof, a mini explosion forming. He shields his hand over his eyes in order to look up, and he can't quite place his finger on what's wrong here. No matter what, there's something that's not right. He finishes putting on his helmet before meeting Chief Boden over at the sideline, standing next to Severide.

"Otis, Hermann, Casey… go into the house and vent out the windows. Severide and the rest of your crew, follow closely in behind them. I don't want anyone getting hurt on this one," he says, making reference to Cruz's episode last week where he nearly killed both Otis and Casey. He remembers Darden, Vargas, and then Severide being benched due to injuries. He can't take another one of his men being hurt. He has a bad feeling about this one, especially since Casey looks like he's about to keel over at any moment.

He winces when hears a harsh cough emanating from Casey and lightly pats his back, feeling the younger man pull away. "I'm fine, Chief," he explains for the billionth time. He's tired of having to explain that. It's not even bad anymore. He will admit that last night worried him a little, but now he's just freezing. But, he's getting ready to walk into a fire, so he thinks that feeling will change sooner than he can imagine.

Casey, Hermann, and Otis make their way inside the burning building. None of them are particularly concerned about this. The house is small, so it won't take long to locate vents. Casey grabs on to his chest, noticing he's wheezing uncontrollably. He fumbles to increase the oxygen in his mask, but then he notices that his two men are watching him closely, so he ignores it. He wants to wrap his arms around himself to try to stop the chills that are wracking his whole body, but he can't. He can feel goose bumps underneath all of his gear.

_We're too late,_ Hermann thinks.

"Holy shit!" Hermann screams. "We gotta get out of here! The floor is gonna cave in!" The extreme temperature of the house just went up multiple notches. His mask start to quiver, and the floor rumbles beneath his feet. The house is too small for this. It can't withstand the heat. Hermann pulls both Otis and Casey closer to the nearest window, breaking it open as quickly as he can.

"Fire department, if anyone is here, call out!" Casey shouts, not forgetting that they came in here to look for survivors as well.

"Help!" is the call he gets only two seconds later. He runs past Hermann and Otis and into the flames, his lungs swelling as he does so. He coughs harshly, rubbing at his chest once again as he rummages through the broken wood in the house. He jumps over a fallen beam and heads into what looks like the remnants of a child's bedroom and immediately spots a little girl laying under her bed. He takes off his mask and places it on her. Without thinking, he picks her up and races toward the window, feeling the floor shake violently as he takes each step. An explosion of fire splits in between where Casey can see the window and his two men waiting for him.

Shit.

And he does what he has to. He shrugs off his jacket, feeling flames brush up against him, and wraps the small child up tightly in it, knowing the fire can't get past this. She's tiny enough to fit completely inside. She sputters and shakes in his arms, and Casey quickly runs through the flames, not realizing what is totally happening. He needs to get the girl out of here. He can see Hermann waiting for him, but he loses his footing. Casey harshly smacks the ground, landing too hard on his right wrist. The immediate pain and shock is too much for him to take. His vision gets blurry, and he…

Hermann grabs the girl and hands her to Otis waiting outside the window before forcing himself to pick up Casey's unconscious body. He's dead weight, barely breathing, and he can feel him trembling all the way through his own gear. As fast as he can, he carries Casey down the ladder, hearing the house crumble behind him. Shay and Dawson are already waiting for Casey and the little girl. Hermann takes off his own mask and watches the two EMTs work on one of his best friends. He knew Casey never should have done this. And now all he can do is watch this happen to him.

* * *

**Author's Note: **I feel like this chapter sucked so much. I'm sorry if you're disappointed in the story now. It will get better. I promise. Anyway, what do you think will happen to Casey? Any other guesses on what he has? Or do you think "nothing's wrong?" I feel like there just wasn't enough action and/or that everyone was completely out of character. Thank you so much for reading, I hope you enjoyed it, and please remember to leave a review!


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